The present invention relates to an electromagnetic wave absorption apparatus of a multi-layer structure and which uses sintered ferrite magnetic bodies, and more particularly to an electromagnetic wave absorption apparatus with broadband characteristics. (Problem to be Solved by the Invention)
An absorber for preventing the reflection of TV waves from buildings and electromagnetic wave darkrooms for the measurement of irradiated electromagnetic waves from electrical apparatus require a favorable absorption of electromagnetic waves over a broad frequency bandwidth. With respect to this, electromagnetic wave absorption bodies which use sintered ferrite have a thickness of 5-8 mm and also has excellent absorption of electromagnetic waves from low frequencies of 30 MHz for example.
FIG. 6 shows the structure of a most fundamental type of ferrite electromagnetic wave absorption body and is configured so that there is an sintered ferrite magnetic body having a thickness d, with a metallic conductor plate behind it (Refer to Hans Wilhelm Helberg "Die Absorption electromagnetischer Wellen in einem grossen Frequenzbereich durch eine duenne homogene Sehicht mit Velusten" Zeitschrifit fuer angewandte Physik, XIII Band Heft 5-1961, p. 237-245; Suetake et al. "Magnetic-type resistance film absorption barriers" Electronic Communications Society Microwave Research Association, 1967.1; Japanese Patent Publication No. 26143-1968.) When the magnetic field reflector coefficient of the surfaces of the ferrite magnetic bodies F in these configurations is made s, then the power absorption coefficient of the electromagnetic wave absorption body is 1-.vertline.s.vertline..sup.2. Accordingly, there is more favorable absorption for the smaller the value of .vertline.s.vertline.. In normal cases, .vertline.s.vertline..ltoreq.0.1 as a guide, or more specifically, coefficient of absorption .gtoreq.0.99 is used.
FIG. 7 shows the absorption characteristics of the electromagnetic wave absorption body shown in FIG. 6, when the frequency f is on the horizontal axis, and the coefficient of reflectivity .vertline.s.vertline. is on the vertical axis. In this case, when the lower of the two frequencies for which .vertline.s.vertline.=0.1 is f1 and the higher is fh, then the frequency band B for which .vertline.s.vertline.=0.1 is satisfied becomes EQU B=fh-f1
This frequency band B has the following relationship with the materials that are used to realize the electromagnetic wave absorption body.
(a) When f1 is to become 30 MHz, the ferrite which is used is of the sintered type and is therefore of an NiZn or MnZn system. The value for fh becomes 300-400 MHz using such a system.
(b) When f1 is to become 90 MHz, the ferrite which is used is also of the sintered type in this case, fh becomes 350-520 MHz.
Of these, an absorber described (a) assumes an absorber of an electromagnetic wave darkroom and so fh=1000 MHz with respect to f1=30 MHz but it is not possible to satisfy this requirement. In addition, with (b), an absorber so that walls of a building can absorb television waves is assumed and f1=90 MHz and fh=800 MHz are assumed, but it is also not possible to satisfy this (Refer to Naito et al. "Ferrite absorbers with broader bands" Electronic Communications Society, Microwave Research Association Japan 1968.3; "Die breitbandige Absorption electromagnetischer Wellen durch duenne Ferritschichten" Zeitschrifit fuer angewandte Physik, XIX Band Heft 6-1965, p.509-514; Japanese Patent Laid Open Application No. 101605-1989) and so the ferrite F shown in FIG. 6 is divided into the two portions F1 and F2 shown in the FIG. 8, and having the respective thicknesses d1 and d2, with a metal reflector plate being attached to the one portion d1, and the other portion d2 being placed apart at the interval Po. This interval Po is filled with air.
According to this configuration, it is possible to satisfy requirements for fh=1000 MHz for f1=30 MHz, and fh=800 MHz for f1=90 MHz. The ferrite F1 and F2 either have the same characteristics, or they can be slightly different. Sintered ferrite having a magnetic permeability of approximately 500 is used when ferrite having the same characteristics is used, and sintered ferrite having a magnetic permeability of approximately 500 is used for F1, and sintered ferrite having a magnetic permeability of approximately 200 is used for F2 so that the overall characteristics are roughly the same as for when the same material is used (Refer to Naito et al. "Ferrite absorbers with broader bands" Electronic Communications Society, Microwave Research Association 1968.3.)
Improved absorbers have not been used for the following reasons. The first is that having both F1 and F2 as sintered ferrite increases the cost, since the number of sintered materials doubles when the required area is configured as in this method.
FIG. 9 shows a conventional example of an absorber having a broader band, where a dielectric body D is inserted between a metal conductor plate C and a ferrite body F. In this case, it is possible to obtain f1=30 MHz and fh=1000 MHz (Refer to Hans Wilhelm Helberg, "Die Absorption electromagnetischer Wellen in einem duenne Materialschieht in Kleinem Abstand vor einer Metallfaeche" Zeitschrifit fuer angewandte Physik, XVI Band Heft 4-1963, p.214-220; Japanese Patent Publication No. 4423-1975; U.S. Pat. No. 3,754,225, Aug. 21, 1973; Japanese Patent Laid Open Application No. 35797-1990; Hashimoto et al., "Practical Design of simple, compact electromagnetic wave darkrooms using ferrite' Shingakuron, Vol. J73-B No. 8, p.421-431 [1990-08]; and S. Abdulah Mirtaheri et al. Widening the Bandwidth of Ferrite Absorbing Wall by Adding a Dielectric Layer" 1991 Electronic Information Communications Society, Shunki Zenkoku Taikai B-290.).
A frequency of 1000 MHz is the current maximum frequency fh, but in the future, when the operating frequencies of electronic apparatus, such as the clock frequencies of personal computers become higher, the electromagnetic waves which are generated by and irradiated from such apparatus will have higher frequencies and fh will become higher than 1000 MHz. (Summary of the invention)
In the light of the problems described above, the present invention has as an object the provision of an electromagnetic wave absorbing apparatus having a broadband electromagnetic wave absorbing characteristic, and which can also be used for the improvement of existing electromagnetic wave absorbing apparatus.